Silver articles turn black when kept in the open for a longer time due to the formation of a compound called silver sulfide (Ag2S).

Option 1: H2S is hydrogen sulfide, a gas with a strong odor similar to rotten eggs. While it can cause silver to tarnish, it does not directly form a black compound on the surface of the silver.

Option 2: AgS is silver sulfide, which is the correct compound responsible for the blackening of silver articles. This compound is formed when silver reacts with sulfur in the air.

Option 3: AgSO4 is silver sulfate, a different compound that is not responsible for the blackening of silver articles in the open.

Option 4: Ag2S is the correct answer. It refers to silver sulfide, the compound that forms on the surface of silver when it reacts with sulfur in the air. This compound gives silver articles a black or darkened appearance.

Therefore, the correct answer is option 4, Ag2S.